Field of the Invention
This invention relates generally to the field of charge air boosting of internal combustion engines and more particularly to a variable geometry turbine employing balanced variable vanes and a hydraulic servo driven by a proportional solenoid for vane closing with a reaction spring for vane opening.
Description of the Related Art
Variable Geometry Turbines (VGT) for turbochargers have become very popular in various automotive and truck market segments over the past 20 years. All major turbocharger manufacturers now have VGTs in production. The capability of the VGT to increase the controllability of the air flow to the engine has proven to be of great value to certain engine market segments. The VGT can be used to vary the air fuel ratio (AF), improve the transient response of the turbocharger, provide additional back pressure to the engine to drive external cooled EGR, and/or provide additional back pressure to augment decompression braking.
This increased capability does have certain drawbacks. For example increased turbocharger cost, decreased reliability, reduced durability, additional noise, more difficult packaging of the turbocharger in the engine compartment, and increased complexity in troubleshooting.
There have been attempts to mitigate some of these negative factors with a reduced cost design by using fewer and simpler parts which can be made using net-shape technologies, and integrated actuation. The integrated actuation improves the packaging by reducing the total volume, and reduces the number of failure modes related to non-integrated, bolt on actuation.
Electro-hydraulic actuation as used on embodiments disclosed in U.S. Pat. No. 6,269,642 issued on Aug. 7, 2001, eliminates the need for external power to be brought to the turbocharger and instead uses the oil system supplied for lubrication and damping as a power source. An additional advantage to this is that it carries heat away rather than creating more heat as an electrical actuation system would.
However, a number of problems still exist with this type of design. To achieve a simple one piece net shape vane, the actuation system was incorporated in the flow passage. This increases the temperature, level of contaminants, and disrupts the aerodynamic flow, thus leading to reduced durability. The unique vane/unison ring conjugate action limited the rotation of the vanes to about 20 degrees.
A double acting hydraulic servo with four way spool valve and proportional solenoid has a number of issues as well. Due to the double acting nature, a spool valve has to have a number of nested annular passages which results in small flow areas. This limits the response of the system and makes it highly dependent on the oil viscosity. With cold starts in frigid climates, this system will take minutes to become functional with appropriate response times. Another issue with a double acting system is that it is more costly. Twice as many control interfaces are required and the precision required of the system is very high.
The vane mechanism is the heart of the variable geometry turbine, and is the most expensive and most difficult to produce cost effectively with high reliability. Initial designs in the industry had many small, very precise parts which were made from super-alloys and machined or ground to very fine tolerances. The majority of the designs used a multi-piece vane/arm/lever that had to be welded during assembly. These designs were very costly and also resulted in cantilevered loads, where the aerodynamic torque on the vane had to be resisted by a force out of the plane of the vane or the support bearings. These complex cantilevered loadings result in much higher loads, particularly at the ends of the “bearings” or supports. Since the vane mechanism has to move billions of times in its life, live in a high vibration environment, survive temperatures above 700 C, in a oxidizing atmosphere, with no lubrication and no sticking or binding, additional loads trying to cock the vane in the bearing are very problematic.
It is therefore desirable to provide a variable geometry turbocharger which overcomes the shortcomings of the prior art to allow lower cost, higher durability and reliability, expanded range flexibility and improved response under all ambient conditions.